In order to reduce the harmonic distortion of the output voltage of power electronic converters, so called multilevel converters based on cell strings, providing an output voltage which may assume several discrete levels, have been proposed. A cell string typically comprises a plurality of series-connected converter cells, where each cell contains a number of switching elements, e.g., transistors, and an energy storage element in form of a capacitor, connected either in a half-bridge or a full-bridge type arrangement.
A converter for converting between direct current (DC) voltages is herein denoted a DC-DC converter. DC-DC converters are known which are based on the concept of the above described cascaded, i.e., series-connected, converter cells. Typically, such a converter 100, as illustrated in FIG. 1, is based on a transformer 101 and two strings 102-105 of converter cells on each side of transformer 101. Each string 102-105 of converter cells is arranged for providing a DC voltage towards its respective DC terminal 106/107, and an alternating current (AC) voltage towards transformer 101. Transformer 101 allows to adjust the AC voltage levels, which, in turn, results in an offset of the DC voltages between the two terminals 106 and 107. However, due to the transformer, such solutions suffer from additional power losses. Further, the transformer incurs extra costs.